There are four basic stances- standing, sitting, crouching (or kneeling) and prone. They all have good points and bad points, but you will need to know them all. Incidentally, all stances have different recoil characteristics, so there is little point in zeroing from prone position if you tend to hunt from a standing position. That said, I prefer to zero firstly from the prone or sitting positions, then move to standing position (which I usually hunt from) and finalise my zero. (N.B- these are the stances that work for me. You may find that you have to alter them slightly to your style of hunting or situation, or even to your strength/ stature etc.) When moving into any position, you should wait for your breathing and heart rate to settle down before firing.
This stance is the most commonly used stance in hunting. It is not the most stable of stances, and so should not be held for too long- ideally, less than five seconds. I.e. quarry is spotted, crosshairs placed on target, half breathe out and fire shot. If the rifle is held too long on target, muscle shakes will occur, especially if your combination is heavy (although this occurs in some degree whatever the stance). The standing stance is useful because it can always be assumed under any conditions, and it also gives good elevation to see quarry for example over a rise in the ground. The standing stance is best assumed as follows:
Place your feet about shoulder width apart at 90 degrees to your target. Raise the rifle butt into your right shoulder (or vice versa if left handed), and hold it there with the left hand on the forestock, pulling the gun into your shoulder. Grip the chin down on the cheekpiece of the rifle to lock everything solid, and to bring the sightline from the eye through the centre of the scope optics and onto the target. The trigger finger should be placed on the trigger, and the rest of the trigger hand can be wrapped around the pistol grip of the weapon, though little if any force should be exerted to support the weight of the weapon with this hand. Make sure the eye is not too close to the rear lens of the scope, or recoil could cause it to hit you in the face. The rifle can now be swung about the body, covering at least 60º to either side of the line to your target. If a shot needs to be held for a long time (and this does happen), the left hand can be pulled back along the forestock of the rifle, and the left upper arm and elbow can be rested against the chest to prop the weight of the gun. This does however cause problems due to the effects of heartbeat on the arm resting against the chest.
This is the most stable and accurate of stances, but circumstances mean that there is seldom time to use this stance while hunting- it has more relevance to Field Target shooting.
Sit on the ground, again at 90º or so to your quarry. Bring the left foot close to the left side of the body, and allow the left knee to rise up towards your face. The right foot can either be curled towards the left side of the body (behind the left foot), but keeping the right knee flat to the ground, or it can be simply left to stick out in front of the body with the right knee slightly raised. Either way, it supports the stance. The gun is then shouldered in the same way as above, but the left elbow can be rested on the left knee to support the weight of the gun and give good stability. Reloading is easiest in this stance if the gun is broken with the left hand, and the pellet inserted with the left hand.
This stance is only really used when the standing or sitting stances are impractical (e.g. a branch is in the way of a standing shot, and there is no time to take a sitting position). It is slightly more stable than a standing position due to the lower centre of gravity, but not as stable as a sitting position. It is a hard stance to describe, being mainly taken due to instinct.
Crouch down on your haunches with both knees out in front of you. Shoulder the rifle as described in the standing stance. Your feet will only be in contact with the ground at the front half of your shoes, and you cannot rest an elbow on your knees, so this stance is not a long term stance either.
This stance is very accurate and stable, but is again seldom practical due to time considerations. However, if your stalking is good and you make good use of landscape features, it can be a very useful stance. (For example, if you crawl up to a bank of earth behind which you know rabbits tend to be found, this stance allows you to just poke your rifle and your head over the bank and fire away).
Lie down flat on the ground with the rifle out in front of you. Your quarry should ideally be a little to the left of the line of your body- the rifle and the body should not form a straight line. The left elbow should be placed in front of the body, and the right elbow a little closer to the body. The weight of the rifle and upper body should rest on the two elbows. Again, the left arm will pull the rifle into the shoulder, and the cheek will hold it in place, together with the right hand on the pistol grip. You may find that the right hand will rest against the cheek.
Reloading from this position can be a problem. With practice, the left hand can break the rifle (assuming you are using a springer) and insert the pellet without removing the elbow from the ground (it helps to keep your pellets in a pouch on the rifle or somewhere else handy). Alternatively, you can rise to a crouching position to reload, then lower yourself back to the prone position to fire.